Certain circuits require analog signal processing for operation. Examples include driver circuits for Class D amplifiers. Typically, the driver circuit outputs gating signals for a high and a low side drive. These can be, for example, power devices such as power MOSFET transistors that are coupled to the load. Often, the driver circuit may be provided as an integrated circuit. Additional functions such as over current protection, overload protection, over temperature protection, test functions and other system functions may be included in such an integrated circuit. In a typical application, the driver control circuit receives an analog input signal to be output to a load, and the control circuit also monitors the output to the load with an analog feedback signal that corresponds to the output signal. Feedback control circuitry is used to adjust the high and low driver gating signals output by the control circuit to compensate the circuit for proper operation.
Class D amplifiers are preferred for many applications because the full “on” or full “off” characteristic of the gating signals ensures that the driver portion of the circuit is very efficient. Advantageously, in the Class D amplifier neither the high side nor low side MOSFET driver transistor is operated in the so-called “linear” fashion. The driver transistor gating signals are arranged so that the transistor is either fully ON or OFF. The operation range where the transistor is in the “active” mode, and therefore forms an impedance, is avoided. This approach avoids the heat generating resistive operation that transistors biased in the linear mode produce.
Further, in most typical prior Class D circuits, the control circuitry sets up a short “dead time” between transitions from a point where the high side driver is on, to where the low side driver on. The purpose of the “dead time” is to avoid “shoot through” currents. In shoot through, the high side and low side drivers are both temporarily on, producing a current path from the positive voltage supply to the ground or negative voltage supply terminal; this results in lost power and very inefficient circuit operation. By using a dead time control scheme, the shoot through phenomenon is reduced. The fact that the amplifier transistors are always operated in their most efficient modes may also eliminate the need for expensive and area consuming heat shields and fans to cool the circuit. In contrast, other circuit topologies such as Class A, Class B, or Class A/B may often require these cooling approaches.
The Class D amplifier is particularly often used as a low frequency amplifier due to the high efficiency and low heat dissipation characteristics. A particular low frequency amplifier application is as an audio amplifier. A Class D amplifier in an audio application receives as its signal input an input signal at a frequency in the human audible range, a low frequency time varying signal of not more than 20 kHz. Typically, in a known amplifier circuit, this input is then compared with a much higher frequency signal from a sawtooth or other ramp signal generator. A resulting pulse width modulated (PWM) signal of a square wave form is generated using a comparator. This PWM signal is then used to form the switching signals and the gating signals for the driver transistors. These signals will be of frequency equal to the ramp or sawtooth. This PWM signal is used to form the high and low driver gating signals for the amplifier transistors.
FIG. 1 illustrates in a simplified circuit diagram a Class D audio amplifier of the prior art. In FIG. 1 an audio input source 11 provides a differential voltage input, with the signal in the audible range of approximately 20 Hz-20 kHz. Analog integrator 13 then outputs a differential integrated signal to the comparators 19 and 17 which output a differential pulse width modulated (PWM) signal to the h-bridge, and then to PMOS and NMOS driver circuits in IC 21. In this non-limiting audio amplifier example, the circuit outputs then drive a conventional 8 or 4 ohm speaker 23, for example, as a load. As is known in the art, if a ramp generator 15 is used to drive the comparators, a PWM output signal results with the same frequency as the ramp or sawtooth frequency.
As is known to those skilled it the art, the Class D amplifier is sometimes referred to as a “digital” amplifier, although strictly speaking, that term is not accurate. This merely means that the power transistors in the driver IC 21 in FIG. 1 are operated fully ON or fully OFF.
While the Class D audio amplifier of the prior art is effective, there are several aspects to the prior art circuit that make it unattractive for integration into present day integrated circuits. Analog components such as are required by the analog integrator and the ramp generator of FIG. 1 are difficult to reliably produce with sufficient precision in present day semiconductor manufacturing processes, such as 45 nanometer and smaller minimum feature size semiconductor manufacturing technologies. Further, these advanced ultra deep submicron semiconductor processes are often optimized for manufacturing digital signal level circuits, not precision analog circuitry. The designed-for core supply voltages are quite low; core voltage supply levels used in present day integrated circuits are dropping and are presently around 1 Volt and trending lower. This low supply voltage level makes accurate amplifier design for amplification of a larger voltage swing input signal very difficult. The designed values of passive components such as capacitors and resistors used in precision analog circuitry can be subject to variations in the process, temperature, or other effects. Variations in the values of these components make control of a feedback loop very difficult, and as is known to those skilled in the art, loop control is necessary to maintain stability in such a feedback system to prevent the analog loop control circuit from oscillating, for example.
Thus there is a continuing need for an improved Class D amplifier that is compatible with advanced sub micron semiconductor processes. The circuit should be configured with a minimal amount of required passive or precision analog components, and should have a process and voltage tolerant loop control function. The Class D amplifier circuit should be adaptable so as to avoid circuit reliability problems associated with component value changes due to process variations, and temperature, supply voltages, load current variations or other variances in the environment where the circuit is used.